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<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> - <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new"> <code>new</code> expression</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete"> <code>delete</code> expression</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Classes</a></td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"> Class declaration</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/constructor"> Constructors</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this"> <code>this</code> pointer</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access"> Access specifiers</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"> <code>friend</code> specifier</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual"> Virtual function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"> <code>override</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"> <code>final</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><tt>explicit</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static"><tt>static</tt></a> </td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor"> Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor"> Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect"> Copy assignment</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect"> Move assignment</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor"> Destructor</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"><a href="templates.html" title="cpp/language/templates"> Templates</a> </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> Class template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> Function template </a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> Template specialization</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="parameter_pack.html" title="cpp/language/parameter pack"> Parameter packs</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="history.html" title="cpp/language/history"> History of C++</a></td></tr>
</table></div></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="basic_concepts.html" title="cpp/language/basic concepts"> Basic Concepts</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="../keywords.html" title="cpp/keyword"> Language keywords</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="charset.html" title="cpp/language/charset"> Character sets and encodings</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="translation_phases.html" title="cpp/language/translation phases"> Phases of translation</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../comments.html" title="cpp/comment"> Comments</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="main_function.html" title="cpp/language/main function"> The main() function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="punctuators.html" title="cpp/language/punctuators"> Punctuation</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="name.html" title="cpp/language/identifiers"> Names and identifiers</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="type-id.html" title="cpp/language/type"> Types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="types.html" title="cpp/language/types"> Fundamental types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="objects.html" title="cpp/language/object"> Objects</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="scope.html" title="cpp/language/scope"> Scope</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lifetime.html" title="cpp/language/lifetime"> Object lifetime</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="storage_duration.html" title="cpp/language/storage duration"> Storage duration and linkage</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> Definitions and ODR</strong> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lookup.html" title="cpp/language/lookup"> Name lookup</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="qualified_lookup.html" title="cpp/language/qualified lookup"> Qualified name lookup</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="unqualified_lookup.html" title="cpp/language/unqualified lookup"> Unqualified name lookup</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="as_if.html" title="cpp/language/as if"> The as-if rule</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="ub.html" title="cpp/language/ub"> Undefined behavior</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="memory_model.html" title="cpp/language/memory model"> Memory model</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="modules.html" title="cpp/language/modules"> Modules</a> <span class="t-mark-rev t-since-cxx20">(C++20)</span> </td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p><i>Definitions</i> are <a href="declarations.html" title="cpp/language/declarations">declarations</a> that fully define the entity introduced by the declaration. Every declaration is a definition, except for the following:
</p>
<ul><li> A function declaration without a function body:
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declares, but doesn't define f</span></pre></div></div>
<ul><li> Any declaration with an <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">extern</span></span></span> <a href="storage_duration.html" title="cpp/language/storage duration">storage class specifier</a> or with a <a href="language_linkage.html" title="cpp/language/language linkage">language linkage</a> specifier (such as <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">extern</span> <span class="st0">"C"</span></span></span>) without an initializer:
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">extern</span> <span class="kw4">const</span> <span class="kw4">int</span> a<span class="sy4">;</span>     <span class="co1">// declares, but doesn't define a</span>
<span class="kw4">extern</span> <span class="kw4">const</span> <span class="kw4">int</span> b <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// defines b</span></pre></div></div>
<ul><li> Declaration of a <span class="t-rev-inl t-since-cxx17"><span>non-inline</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span> <a href="static.html" title="cpp/language/static">static data member</a> inside a class definition:
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S
<span class="br0">{</span>
    <span class="kw4">int</span> n<span class="sy4">;</span>               <span class="co1">// defines S::n</span>
    <span class="kw4">static</span> <span class="kw4">int</span> i<span class="sy4">;</span>        <span class="co1">// declares, but doesn't define S::i</span>
    <span class="kw1">inline</span> <span class="kw4">static</span> <span class="kw4">int</span> x<span class="sy4">;</span> <span class="co1">// defines S::x</span>
<span class="br0">}</span><span class="sy4">;</span>                       <span class="co1">// defines S</span>
 
<span class="kw4">int</span> S<span class="sy4">::</span><span class="me2">i</span><span class="sy4">;</span>                <span class="co1">// defines S::i</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<ul><li> <span class="t-mark">(deprecated)</span> Namespace scope declaration of a static data member that was defined within the class with the <code><a href="constexpr.html" title="cpp/language/constexpr">constexpr</a></code> specifier:
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S
<span class="br0">{</span>
    <span class="kw4">static</span> <span class="kw4">constexpr</span> <span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span> <span class="co1">// implicitly inline, defines S::x</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">constexpr</span> <span class="kw4">int</span> S<span class="sy4">::</span><span class="me2">x</span><span class="sy4">;</span> <span class="co1">// declares S::x, not a redefinition</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
<ul><li> Declaration of a class name (by <a href="class.html#Forward_declaration" title="cpp/language/class">forward declaration</a> or by the use of the elaborated type specifier in another declaration):
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S<span class="sy4">;</span>             <span class="co1">// declares, but doesn't define S</span>
 
<span class="kw1">class</span> Y f<span class="br0">(</span><span class="kw1">class</span> T p<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declares, but doesn't define Y and T (and also f and p)</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<ul><li> An <a href="enum_class.html" title="cpp/language/enum">opaque declaration</a> of an enumeration:
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw2">enum</span> Color <span class="sy4">:</span> <span class="kw4">int</span><span class="sy4">;</span> <span class="co1">// declares, but doesn't define Color</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<ul><li> Declaration of a <a href="template_parameters.html" title="cpp/language/template parameters">template parameter</a>:
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> T<span class="sy1">&gt;</span> <span class="co1">// declares, but doesn't define T</span></pre></div></div>
<ul><li> A parameter declaration in a function declaration that isn't a definition:
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// declares, but doesn't define f and x</span>
 
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span>  <span class="co1">// defines f and x</span>
<span class="br0">{</span>
    <span class="kw1">return</span> x <span class="sy2">+</span> a<span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<ul><li> A <a href="typedef.html" title="cpp/language/typedef">typedef</a> declaration:
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">typedef</span> S S2<span class="sy4">;</span> <span class="co1">// declares, but doesn't define S2 (S may be incomplete)</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<ul><li> An <a href="type_alias.html" title="cpp/language/type alias">alias-declaration</a>:
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">using</span> S2 <span class="sy1">=</span> S<span class="sy4">;</span> <span class="co1">// declares, but doesn't define S2 (S may be incomplete)</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<ul><li> A <a href="using_declaration.html" title="cpp/language/using declaration">using-declaration</a>:
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">using</span> N<span class="sy4">::</span><span class="me2">d</span><span class="sy4">;</span> <span class="co1">// declares, but doesn't define d</span></pre></div></div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<ul><li> Declaration of a <a href="deduction_guide.html#Explicit_deduction_guides" title="cpp/language/class template argument deduction">deduction guide</a> (does not define any entities)
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
<tr class="t-rev t-since-cxx11"><td>
<ul><li> A <a href="static_assert.html" title="cpp/language/static assert">static_assert</a> declaration (does not define any entities)
</li><li> An <a href="declarations.html" title="cpp/language/declarations">attribute declaration</a> (does not define any entities)
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<ul><li> An <a href="declarations.html" title="cpp/language/declarations">empty declaration</a> (does not define any entities)
</li><li> A <a href="namespace.html#Using-directives" title="cpp/language/namespace">using-directive</a> (does not define any entities)
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<ul><li> An <a href="class_template.html#Class_template_instantiation" title="cpp/language/class template">explicit instantiation declaration</a> (an "extern template"):
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">extern</span> <span class="kw1">template</span>
f<span class="sy1">&lt;</span><span class="kw4">int</span>, <span class="kw4">char</span><span class="sy1">&gt;</span><span class="sy4">;</span> <span class="co1">// declares, but doesn't define f&lt;int, char&gt;</span></pre></div></div>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<ul><li> An <a href="template_specialization.html" title="cpp/language/template specialization">explicit specialization</a> whose declaration is not a definition:
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;&gt;</span>
<span class="kw1">struct</span> A<span class="sy1">&lt;</span><span class="kw4">int</span><span class="sy1">&gt;</span><span class="sy4">;</span> <span class="co1">// declares, but doesn't define A&lt;int&gt;</span></pre></div></div>
<p>An <a href="asm.html" title="cpp/language/asm">asm declaration</a> does not define any entities, but it is classified as a definition.
</p><p>Where necessary, the compiler may implicitly define the <a href="default_constructor.html" title="cpp/language/default constructor">default constructor</a>, <a href="copy_constructor.html" title="cpp/language/copy constructor">copy constructor</a>, <a href="move_constructor.html" title="cpp/language/move constructor">move constructor</a>, <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect">copy assignment operator</a>, <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect">move assignment operator</a>, and the <a href="destructor.html" title="cpp/language/destructor">destructor</a>.
</p><p>If the definition of any object results in an object of <a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">incomplete type</a> or <a href="abstract_class.html" title="cpp/language/abstract class">abstract class type</a>, the program is ill-formed.
</p><p><span id="ODR"></span>
</p>
<h3><span class="mw-headline" id="One_Definition_Rule">One Definition Rule</span></h3>
<p>Only one definition of any variable, function, class type, enumeration type, <span class="t-rev-inl t-since-cxx20"><span><a href="constraints.html" title="cpp/language/constraints">concept</a></span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span> or template is allowed in any one translation unit (some of these may have multiple declarations, but only one definition is allowed).
</p><p>One and only one definition of every non-<a href="inline.html" title="cpp/language/inline">inline</a> function or variable that is <i>odr-used</i> (see below) is required to appear in the entire program (including any standard and user-defined libraries). The compiler is not required to diagnose this violation, but the behavior of the program that violates it is undefined.
</p><p>For an inline function <span class="t-rev-inl t-since-cxx17"><span>or inline variable</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>, a definition is required in every translation unit where it is <i>odr-used</i>.
</p><p>For a class, a definition is required wherever the class is used in a way that requires it to be <a href="incomplete_type.html" title="cpp/language/incomplete type" class="mw-redirect">complete</a>.
</p><p>There can be more than one definition in a program of each of the following: class type, enumeration type, inline function<span class="t-rev-inl t-since-cxx17"><span>, inline variable</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>, <a href="templates.html#Templated_entity" title="cpp/language/templates">templated entity</a> (template or member of template, but not full <a href="template_specialization.html" title="cpp/language/template specialization">template specialization</a>), as long as all of the following is true:
</p>
<ul><li> each definition appears in a different translation unit
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<ul><li> the definitions are not <a href="modules.html" title="cpp/language/modules">attached to a named module</a>
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx20">(since C++20)</span></td></tr>
</table>
<ul><li> each definition consists of the same sequence of tokens (typically, appears in the same header file)
</li><li> name lookup from within each definition finds the same entities (after overload-resolution), except that
</li></ul>
<dl><dd><ul><li> constants with internal or no linkage may refer to different objects as long as they are not odr-used and have the same values in every definition
</li></ul>
</dd></dl>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<dl><dd><ul><li> lambda-expressions that are not in a default argument<span class="t-rev-inl t-since-cxx20"><span> or a default template argument</span> <span><span class="t-mark-rev t-since-cxx20">(since C++20)</span></span></span> are uniquely identified by the sequence of tokens used to define them
</li></ul>
</dd></dl>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<ul><li> overloaded operators, including conversion, allocation, and deallocation functions refer to the same function from each definition (unless referring to one defined within the definition)
</li><li> the language linkage is the same (e.g. the include file isn't inside an extern "C" block)
</li><li> the three rules above apply to every default argument used in each definition
</li><li> if the definition is for a class with an implicitly-declared constructor, every translation unit where it is odr-used must call the same constructor for the base and members
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx20"><td>
<ul><li> if the definition is for a class with a defaulted <a href="default_comparisons.html" title="cpp/language/default comparisons">three-way comparison</a>, every translation unit where it is odr-used must call the same comparison operator for the base and members
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx20">(since C++20)</span></td></tr>
</table>
<ul><li> if the definition is for a template, then all these requirements apply to both names at the point of definition and dependent names at the point of instantiation
</li></ul>
<p>If all these requirements are satisfied, the program behaves as if there is only one definition in the entire program. Otherwise, the program is ill-formed, no diagnostic required. 
</p><p>Note: in C, there is no program-wide ODR for types, and even extern declarations of the same variable in different translation units may have different types <a href="../../c/language/compatible_type.html#Compatible_types" title="c/language/type">as long as they are compatible</a>. In C++, the source-code tokens used in declarations of the same type must be the same as described above: if one .cpp file defines <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span> S <span class="br0">{</span> <span class="kw4">int</span> x<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span></span></span> and the other .cpp file defines <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span> S <span class="br0">{</span> <span class="kw4">int</span> y<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span></span></span>, the behavior of the program that links them together is undefined. This is usually resolved with <a href="namespace.html" title="cpp/language/namespace">unnamed namespaces</a>.
</p>
<h4><span class="mw-headline" id="ODR-use">ODR-use</span></h4>
<p>Informally, an object is odr-used if its value is read (unless it is a compile time constant) or written, its address is taken, or a reference is bound to it; a reference is odr-used if it is used and its referent is not known at compile time; and a function is odr-used if a function call to it is made or its address is taken. If an object, a reference or a function is odr-used, its definition must exist somewhere in the program; a violation of that is usually a link-time error.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S
<span class="br0">{</span>
    <span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// static data member</span>
    <span class="co1">// a definition outside of class is required if it is odr-used</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&amp;</span> f<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&amp;</span> r<span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw4">int</span> n <span class="sy1">=</span> b <span class="sy4">?</span> <span class="br0">(</span><span class="nu0">1</span>, S<span class="sy4">::</span><span class="me2">x</span><span class="br0">)</span> <span class="co1">// S::x is not odr-used here</span>
          <span class="sy4">:</span> f<span class="br0">(</span>S<span class="sy4">::</span><span class="me2">x</span><span class="br0">)</span><span class="sy4">;</span>  <span class="co1">// S::x is odr-used here: a definition is required</span></pre></div></div>
<p>Formally,
</p>
<div class="t-li1"><span class="t-li">1)</span> a variable <code>x</code> in a <i>potentially-evaluated</i> expression <code>ex</code> is odr-used unless both of the following are true:
<dl><dd><ul><li> applying lvalue-to-rvalue conversion to <code>x</code> yields a constant expression that doesn't invoke non-trivial functions
</li><li> either <code>x</code> is not an object (that is, <code>x</code> is a reference) or, if <code>x</code> is an object, it is one of the <i>potential results</i> of a larger expression <code>e</code>, where that larger expression is either a <a href="expressions.html#Discarded-value_expressions" title="cpp/language/expressions">discarded-value expression</a> or has the lvalue-to-rvalue conversion applied to it
</li></ul>
</dd></dl>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span> <span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// applying lvalue-to-rvalue conversion</span>
                                      <span class="co1">// to S::x yields a constant expression</span>
 
<span class="kw4">int</span> f<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span> 
    S<span class="sy4">::</span><span class="me2">x</span><span class="sy4">;</span>        <span class="co1">// discarded-value expression does not odr-use S::x</span>
 
    <span class="kw1">return</span> S<span class="sy4">::</span><span class="me2">x</span><span class="sy4">;</span> <span class="co1">// expression where lvalue-to-rvalue conversion</span>
                 <span class="co1">// applies does not odr-use S::x</span>
<span class="br0">}</span></pre></div></div></div>
<div class="t-li1"><span class="t-li">2)</span> <a href="this.html" title="cpp/language/this"><code>*this</code></a> is odr-used if <span class="t-c"><span class="mw-geshi cpp source-cpp">this</span></span> appears as a <i>potentially-evaluated</i> expression (including the implicit <span class="t-c"><span class="mw-geshi cpp source-cpp">this</span></span> in a non-static member function call expression)</div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx17"><td>
<div class="t-li1"><span class="t-li">3)</span> A <a href="structured_binding.html" title="cpp/language/structured binding">structured binding</a> is odr-used if it appears as a <i>potentially-evaluated</i> expression.
</div></td>
<td><span class="t-mark-rev t-since-cxx17">(since C++17)</span></td></tr>
</table>
<p>In the definitions above, <i>potentially-evaluated</i> means the expression is not an <a href="expressions.html#Unevaluated_expressions" title="cpp/language/expressions">unevaluated operand</a> (or its subexpression), such as the operand of <a href="sizeof.html" title="cpp/language/sizeof"><code>sizeof</code></a> and <i>a set of potential results</i> of an expression <span class="t-c"><span class="mw-geshi cpp source-cpp">E</span></span> is a (possibly empty) set of id-expressions that appear within <span class="t-c"><span class="mw-geshi cpp source-cpp">E</span></span>, combined as follows:
</p>
<ul><li> If <span class="t-c"><span class="mw-geshi cpp source-cpp">E</span></span> is an <a href="expressions.html#Primary_expressions" title="cpp/language/expressions">id-expression</a>, the expression <span class="t-c"><span class="mw-geshi cpp source-cpp">E</span></span> is its only potential result.
</li><li> If <span class="t-c"><span class="mw-geshi cpp source-cpp">E</span></span> is a subscript expression (<span class="t-c"><span class="mw-geshi cpp source-cpp">E1<span class="br0">[</span>E2<span class="br0">]</span></span></span>) where one of the operands is an array, the potential results of that operand is included in the set.
</li><li> If <span class="t-c"><span class="mw-geshi cpp source-cpp">E</span></span> is a class member access expression of the form <span class="t-c"><span class="mw-geshi cpp source-cpp">E1.<span class="me1">E2</span></span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp">E1.<span class="kw1">template</span> E2</span></span> naming a non-static data member, the potential results of <span class="t-c"><span class="mw-geshi cpp source-cpp">E1</span></span> is included in the set.
</li><li> If <span class="t-c"><span class="mw-geshi cpp source-cpp">E</span></span> is a class member access expression naming a static data member, the id-expression designating the data member is included in the set.
</li><li> If <span class="t-c"><span class="mw-geshi cpp source-cpp">E</span></span> is a pointer-to-member access expression of the form <span class="t-c"><span class="mw-geshi cpp source-cpp">E1.<span class="sy2">*</span>E2</span></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp">E1.<span class="sy2">*</span><span class="kw1">template</span> E2</span></span> whose second operand is a constant expression, the potential results of <span class="t-c"><span class="mw-geshi cpp source-cpp">E1</span></span> are included in the set.
</li><li> If <span class="t-c"><span class="mw-geshi cpp source-cpp">E</span></span> is an expression in parentheses (<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span>E1<span class="br0">)</span></span></span>), the potential results of <span class="t-c"><span class="mw-geshi cpp source-cpp">E1</span></span> are included in the set.
</li><li> If <span class="t-c"><span class="mw-geshi cpp source-cpp">E</span></span> is a glvalue conditional expression (<span class="t-c"><span class="mw-geshi cpp source-cpp">E1 <span class="sy4">?</span> E2 <span class="sy4">:</span> E3</span></span>, where <span class="t-c"><span class="mw-geshi cpp source-cpp">E2</span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp">E3</span></span> are glvalues), the union of the potential results of <span class="t-c"><span class="mw-geshi cpp source-cpp">E2</span></span> and <span class="t-c"><span class="mw-geshi cpp source-cpp">E3</span></span> are both included in the set.
</li><li> If <span class="t-c"><span class="mw-geshi cpp source-cpp">E</span></span> is a comma expression (<span class="t-c"><span class="mw-geshi cpp source-cpp">E1, E2</span></span>), the potential results of <span class="t-c"><span class="mw-geshi cpp source-cpp">E2</span></span> are in the set of potential results.
</li><li> Otherwise, the set is empty.
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> S
<span class="br0">{</span>
    <span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> a <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
    <span class="kw4">static</span> <span class="kw4">const</span> <span class="kw4">int</span> b <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">bool</span> x<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">return</span> x <span class="sy4">?</span> S<span class="sy4">::</span><span class="me2">a</span> <span class="sy4">:</span> S<span class="sy4">::</span><span class="me2">b</span><span class="sy4">;</span>
    <span class="co1">// x is a part of the subexpression "x" (to the left of ?),</span>
    <span class="co1">// which applies lvalue-to-rvalue conversion, but applying that conversion to x</span>
    <span class="co1">// does not yield a constant expression, so x is odr-used</span>
    <span class="co1">// S::a and S::b are lvalues, and carry over as "potential results"</span>
    <span class="co1">// to the result of the glvalue conditional</span>
    <span class="co1">// That result is then subject to lvalue-to-rvalue conversion requested</span>
    <span class="co1">// to copy-initialize the return value, therefore S::a and S::b are not odr-used</span>
<span class="br0">}</span></pre></div></div>
<div class="t-li1"><span class="t-li">4)</span> A function is odr-used in following cases:
<ul><li> A function is odr-used if it is named by (see below) a potentially-evaluated expression or conversion.
</li><li> A <a href="virtual.html" title="cpp/language/virtual">virtual member function</a> is odr-used if it is not a pure virtual member function (addresses of virtual member functions are required to construct the vtable).
</li><li> A non-placement allocation or deallocation function for a class is odr-used by the definition of a constructor of that class.
</li><li> A non-placement deallocation function for a class is odr-used by the definition of the destructor of that class, or by being selected by the lookup at the point of definition of a virtual destructor.
</li><li> An assignment operator in a class <code>T</code> that is a member or base of another class <code>U</code> is odr-used by an implicitly-defined copy-assignment or move-assignment functions of <code>U</code>.
</li><li> A constructor (including default constructors) for a class is odr-used by the <a href="initialization.html" title="cpp/language/initialization">initialization</a> that selects it.
</li><li> A destructor for a class is odr-used if it is potentially invoked.</li></ul></div>

<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br>Reason: list of all situations where odr-use makes a difference </td></tr></table>
<h4><span class="mw-headline" id="Naming_a_function">Naming a function</span></h4>
<p>A function is named by an expression or conversion in following cases:
</p>
<ul><li> A function whose name appears as an expression or conversion (including named function, overloaded operator, <a href="cast_operator.html" title="cpp/language/cast operator">user-defined conversion</a>, user-defined placement forms of operator new, non-default initialization)  is named by that expression if it is selected by overload resolution, except when it is an unqualified pure virtual member function or a pointer-to-member to a pure virtual function.
</li><li> An <a href="../memory/new/operator_new.html" title="cpp/memory/new/operator new">allocation</a> or <a href="../memory/new/operator_delete.html" title="cpp/memory/new/operator delete">deallocation</a> function for a class is named by a <a href="new.html" title="cpp/language/new">new expression</a> appearing in an expression.
</li><li> A deallocation function for a class is named by a <a href="delete.html" title="cpp/language/delete">delete expression</a> appearing in an expression.
</li><li> A constructor selected to copy or move an object is considered to be named by the expression or conversion even if <a href="copy_elision.html" title="cpp/language/copy elision">copy elision</a> takes place. <span class="t-rev-inl t-since-cxx17"><span>Using a prvalue in some contexts  does not copy or move an object, see <a href="copy_elision.html#Mandatory_elision_of_copy.2Fmove_operations" title="cpp/language/copy elision">Mandatory elision</a>.</span> <span><span class="t-mark-rev t-since-cxx17">(since C++17)</span></span></span>
</li></ul>
<p>A potentially evaluated expression or conversion odr-uses a function if it names it.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p>A potentially constant evaluated expression or conversion that names a constexpr function makes it <a href="constant_expression.html#Functions_and_variables_needed_for_constant_evaluation" title="cpp/language/constant expression">needed for constant evaluation</a>, which triggers definition of a defaulted function or instantiation of a function template specialization, even if the expression is unevaluated.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/261.html">CWG 261</a>
</td>
<td> C++98
</td>
<td> a deallocation function for a polymorphic class<br>might be odr-used even if there were no relevant<br>new or delete expressions in the program
</td>
<td> supplemented the<br>odr-use cases to cover<br>constructors and destructors
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/678.html">CWG 678</a>
</td>
<td> C++98
</td>
<td> an entity could have definitions<br>with different language linkages
</td>
<td> the behavior is<br>undefined in this case
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1472.html">CWG 1472</a>
</td>
<td> C++98
</td>
<td> reference variables which satisfy the requirements for<br>appearing in a constant expression were odr-used even<br>if the lvalue-to-rvalue conversion is immediately applied
</td>
<td> they are not odr-used<br>in this case
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1614.html">CWG 1614</a>
</td>
<td> C++98
</td>
<td> taking address of a pure virtual function odr-used it
</td>
<td> the function is not odr-used
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1741.html">CWG 1741</a>
</td>
<td> C++98
</td>
<td> constant objects that are immediately lvalue-to-rvalue<br>converted in potentially-evaluated expressions were odr-used
</td>
<td> they are not odr-used
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1926.html">CWG 1926</a>
</td>
<td> C++98
</td>
<td> array subscript expressions didn't propagate potential results
</td>
<td> they propagate
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2300.html">CWG 2300</a>
</td>
<td> C++11
</td>
<td> lambda expressions in different translation<br>units could never have the same closure type
</td>
<td> the closure type can be the<br>same under one definition rule
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2353.html">CWG 2353</a>
</td>
<td> C++98
</td>
<td> a static data member was not a potential result<br>of a member access expression accesing it
</td>
<td> it is
</td></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/2433.html">CWG 2433</a>
</td>
<td> C++14
</td>
<td> a variable template could not have multiple definitions in a program
</td>
<td> it can
</td></tr></table>
<h3><span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-c++20">
<ul><li> C++20 standard (ISO/IEC 14882:2020): 
</li></ul>
<dl><dd><ul><li> 6.3 One definition rule [basic.def.odr] 
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++17">
<ul><li> C++17 standard (ISO/IEC 14882:2017): 
</li></ul>
<dl><dd><ul><li> 6.2 One definition rule [basic.def.odr] 
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++14">
<ul><li> C++14 standard (ISO/IEC 14882:2014): 
</li></ul>
<dl><dd><ul><li> 3.2 One definition rule [basic.def.odr] 
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++11">
<ul><li> C++11 standard (ISO/IEC 14882:2011): 
</li></ul>
<dl><dd><ul><li> 3.2 One definition rule [basic.def.odr] 
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++03">
<ul><li> C++03 standard (ISO/IEC 14882:2003): 
</li></ul>
<dl><dd><ul><li> 3.2 One definition rule [basic.def.odr] 
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c++98">
<ul><li> C++98 standard (ISO/IEC 14882:1998): 
</li></ul>
<dl><dd><ul><li> 3.2 One definition rule [basic.def.odr] 
</li></ul>
</dd></dl>
</div>

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